MESSENGER snapped these 36 images as part of a mosaic revealing the crescent Mercury seen by the spacecraft as it approached. MESSENGER acquired the lower left image first, some 55 minutes before the spacecraft swooped within 125 miles (200 km) of the surface. The name under each of these context images represents the time elapsed (in seconds) since the mission's launch.

NASA/JHUAPL/CIW

March 11, 2008Thirty-three years ago, the Mariner 10 spacecraft made its third and final flyby of Mercury. Most of what we know about the innermost planet came from those three tours. The problem: Due to a quirk of its orbital geometry, Mariner 10 imaged just 45 percent of Mercury.

The MESSENGER spacecraft is beginning to fill in the rest of the picture. MESSENGER — short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging — zipped past Mercury January 14. It didn't zip quite the way scientists anticipated, however. On Monday, at the 39th Lunar and Planetary Science Conference in Houston, David Smith of NASA's Goddard Space Flight Center announced that Mercury perturbed MESSENGER's path more than expected. The result means Mercury's mass has a slightly different distribution than what scientists deduced from Mariner 10.

Smith's was one of more than a dozen reports on MESSENGER delivered at the conference. Louise Prockter of Johns Hopkins University Applied Physics Laboratory described the imaging results. She says MESSENGER imaged 21 percent of the planet that hadn't been seen before, raising the total observed to 66 percent.

Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian Institution's National Air and Space Museum confirmed preliminary MESSENGER findings that "lobate scarps" — cliffs created by crustal faulting — are Mercury's dominant tectonic feature. One scarp, dubbed "Beagle Rupes," extends more than 375 miles (600 kilometers). The new images also show many of the scarps seen by Mariner 10 are longer than thought.

Watters also said the longest lobate scarps concentrate in Mercury's southern hemisphere, and MESSENGER seems to have found slightly more such scarps in equal-sized areas than did Mariner 10. Watters still thinks the scarps formed when Mercury contracted early in its history — but that it might have contracted more than scientists originally believed.

As the MESSENGER team tries to unravel Mercury's many mysteries, they await another big dose of data. In October, the spacecraft returns to Mercury for another visit, and will see the opposite hemisphere on display in January. These and a third flyby in September 2009 will serve as prelude to when MESSENGER begins orbiting Mercury in March 2011. Expect to hear plenty more about the still-enigmatic inner planet for years to come.